A known rotation angle detecting unit rotatably supports a shaft on a housing through a rolling bearing, and includes a magnetic rotation angle sensor mounted in the housing. The magnetic rotation angle sensor comprises a magnetic drum fixed to one end surface of the shaft, and a magnetic sensor for detecting a change in magnetic flux as the magnetic drum rotates and calculating the relative rotation angle. This type of rotation angle detecting unit is used with the shaft coupled to one of the mating members such as a rotary shaft and a sensor case mounted to the other mating member.
Assembling errors are unavoidable between the respective ones of the rotation angle sensor, circuit board, shaft and housing. Thus some of today's magnetic sensors have the function of correcting the measured value based on initial calibration parameters stored for correcting measurement errors resulting from assembling errors. In order to specify the initial calibration parameters, it is necessary to perform actual measurements. Thus, output signals from the magnetic sensor are transmitted to an external calibration device to calculate initial calibration parameters in the external calibration device, and the thus calculated initial calibration parameters are written into nonvolatile semiconductor memory of the magnetic sensor (as disclosed in JP Patent Publication 2004-191101A). Among them, there is known the type in which an external calibration device is connected to the sensor power source connecting end, grounded end and signal output ends of the circuit board carrying the magnetic sensor, and the initial calibration parameters are written while supplying a calibration voltage, which is higher than ordinary use voltage, through the sensor power source connecting end. With this magnetic sensor, since the calibration voltage and the use voltage are both supplied through the sensor power source connecting end, it is not necessary to provide the circuit board with a power source connecting end for calibration, which makes it possible to reduce the size of the circuit board and to simplify its structure.
When, as described above, use voltage and calibration voltage are supplied to the magnetic sensor through the common sensor power source connecting end, it is impossible to mount a voltage stabilizing circuit between the input end of the magnetic sensor and the sensor power source connecting end, because it becomes impossible to supply calibration voltage to the input end of the magnetic sensor. If the voltage stabilizing circuit is not mounted, if the power source voltage supplied is unstable according to the use environment, such as in the case in which the generator of e.g. a construction machine or an industrial machine is used as the external power source, or if the power source voltage supplied cannot be determined to be one, it is difficult to use the rotation angle detecting unit.
Therefore the applicant proposed (in JP Patent Application 2007-23178) a rotation angle detecting unit which comprises a housing including a housing body having mounting openings at first and second ends thereof, respectively, and a lid covering the mounting opening at the first end, a shaft having its portion at the first end received in the housing and its portion at the second end protruding from the mounting opening of the housing at the second end, a rolling bearing rotatably supporting the shaft on the housing body, a magnetic drum fixed to an end surface of the shaft at the first end, and a circuit board carrying a magnetic sensor having the above-described correcting function, wherein a board internal mounting portion is provided on a surface of the lid at the second end for mounting the circuit board, wherein the lid is configured to apply a preload to the rolling bearing by pressing an outer race of the rolling bearing and to position the magnetic sensor relative to the magnetic drum when the lid is fixed in position, wherein a voltage stabilizing circuit for stabilizing and outputting use voltage and a wiring are provided parallel to each other between the input end of the magnetic sensor and the sensor power source connecting end of the circuit board, and a switching portion is provided which can disconnect the wiring, and wherein an operating port is formed in the housing through which the switching portion is accessible for disconnection.
With this rotation angle detecting unit, it is possible to supply calibration voltage to the input end of the sensor through the sensor power source connecting end of the circuit board and the parallel connected wiring. By disconnecting the wiring after writing the initial calibration parameters, it is possible to supply use voltage to the input end of the sensor through the voltage stabilizing circuit. Also, with this rotation angle detecting unit, by pushing the outer race of the rolling bearing with the lid and applying a preload to the bearing, the rigidity of the bearing increases. This prevents displacement of the shaft due to loads applied thereto and shifting between the magnetic drum and the magnetic sensor. Thus it is possible to detect the rotation angle in an optimum manner with the shaft coupled to the above-described rotary shaft and loads applied to the rotary shaft. Since the lid is used, it is not necessary to provide a dedicated cover for mounting the circuit board. The initial calibration parameters can be determined by fixing the lid.
But if the voltage difference between the input voltage value and output voltage value to and from the to the voltage stabilizing circuit (use voltage values) is large, it is necessary to use large-sized power source stabilizing element and resistor have to be used due to generated calorie. The magnetic gap between the magnetic sensor and the magnetic drum is extremely small in size compared to the power source stabilizing element. Thus, if the power source stabilizing element is mounted on the same side as the magnetic sensor, the power source stabilizing element cannot be fitted between the end surface of the shaft at the first end and the circuit board. If the power source stabilizing element is displaced from the end surface of the shaft at the first end, it can be mounted in position. But this increases the outer circumferential length of the circuit board, thus increasing the diameter of the lid and thus the size of the unit. Even if the above-mentioned voltage difference is small, if the power source stabilizing element and the like are mounted on the surface of the circuit board at the second end, since an increased number of electronic parts are mounted on this side, the area of the circuit board increases, thus increasing the diameter of the lid.
If the magnetic sensor is mounted on the surface of the circuit board at the second end and the voltage stabilizing circuit is mounted on the surface of the circuit board at the first end, the unit can be made compact in size. But in this case, the contact area between the circuit board and the surface of the lid at the second end decreases, thus making it difficult to stably mount the circuit board.